1. Field of the Invention
The present invention relates to a terminal adapter for connecting a terminal to a computer local area network capable of identifying any of several terminal types. It relates in particular to terminals adapted to be supplied with power locally and to receive a remote power feed via a connection to a computer local area network, for example an Ethernet network.
2. Description of the Prior Art
Conventionally, terminals connected to a computer local area network (for example personal computers, printers, etc.) are supplied with power locally, by the mains power supply. 110 V or 220 V power supply cords independent of the data connections are therefore used to supply power to the terminals. This solution makes it more difficult to install the local area network:                The use of two cables gives rise to problems of congestion, which can also limit the freedom of movement of persons.        It creates electrical hazards.        
In the case of a telephone, a local power supply from the mains power supply has the further disadvantage that the telephone is unserviceable in the event of a mains power outage, in particular in the event of a fire or natural disaster. This is why conventional telephones receive a remote power feed from their local exchange, which includes back-up batteries.
Telephone terminals connected to a computer local area network are coming into use. It is therefore desirable that some terminals connected to a computer local area network receive a remote power feed via the connection used to send/receive data. One way to transmit a remote power feed current is to use two of the eight wires of the connection: four other wires from those eight wires constitute two pairs of wires respectively used to transmit and receive data. Another method, known as the phantom circuit method, connects the two terminals of a power supply generator in the remote power feed device to the respective center-taps of a winding of a transformer connected to the data receiver pair and a winding of another transformer connected to the data sender pair. At the terminal end, the supply voltage is provided by the respective center-taps of a winding of a transformer connected to the data receiver pair and a winding of another transformer connected to the data sender pair.
In both cases, providing a remote power feed to a terminal via the computer local area network has the disadvantage that the remote power feed device powers a terminal “blind”. The RJ45 connector at the end of the connection can be plugged into a terminal of a type other than a telephone (for example a personal computer, a printer, etc.). There is a risk of damaging the electrical circuits of that terminal. The RJ45 connector of a terminal is generally used in the following manner:                Four of the eight wires are divided into two pairs for respectively transmitting and receiving data. The terminal includes a transformer having a winding connected to the receiver pair and a transformer having a winding connected to the sender pair, each of these windings having a center-tap that can be connected to a reference potential via a low-value resistor.        Four other, unused wires are grounded, often via a combination of resistors and capacitors, to eliminate any crosstalk currents induced by the data signals circulating in the first four wires and to reduce undesirable electromagnetic emissions. If a relatively high supply voltage, for example 48 volts, is applied to this combination of resistors and capacitors, or to the resistors connected to the center-taps of the transformers, the current flowing in the resistors can destroy them.        
Methods of providing a remote power feed to a terminal in a computer local area network avoiding all risk of damage if a terminal that is not adapted to receive a remote power feed from the network is connected thereto are currently under investigation.
One Such Method Entails:                producing a test signal, or a plurality of test signals, on at least two conductors of a connection intended to connect the local area network to a remote terminal, the signal(s) having an energy such that the terminal cannot be damaged under any circumstances;        detecting the presence of a remote terminal adapted to receive a remote power feed by detecting the presence of an identifier circuit placed in the remote terminal, shunting the connection, on the basis of the current created by the test signal(s) in the connection; and        sending a remote power feed over the connection if and only if the presence of a terminal adapted to receive a remote power feed is detected.        
A method of this kind avoids all risk to the terminals because the remote power feed current is sent only after identifying the terminal as one adapted to receive a remote power feed. The intensity and duration of the test signal(s) are chosen so that the operation of detecting the presence of a terminal adapted to receive a remote power feed, before sending the remote power feed, cannot cause any damage if the terminal is not a terminal adapted to receive a remote power feed.
Also, the remote power feed must be interrupted as soon as the terminal is disconnected from the connection, as another terminal could be connected to it at any time. There is therefore provision for continuously detecting the presence of a terminal during remote power feeding, by measuring the remote power feed current. The remote power feed device concludes that a disconnection has occurred if it detects that the current drawn is less than a predetermined threshold value during a time interval of predetermined duration which is sufficient to be sure that a disconnection has in fact taken place.
There is as yet no standard defining the interface between terminals adapted to receive a remote power feed and an Ethernet local area network, but terminals of this kind are already commercially available and included in installed networks. Each manufacturer uses its own interface; in particular, each manufacturer uses a different impedance to identify a given type of terminal and uses different conductors from the eight conductors of a connection, for example an Ethernet connection, to convey the remote power feed. It is not practicable to modify existing terminals to adapt them to the interface used by another manufacturer, and it is therefore not possible at present to mix in the same local area network terminals from different manufacturers adapted to receive a remote power feed.
The object of the invention is to enable mixing in the same local area network of terminals from different manufacturers adapted to receive a remote power feed.